1. Field of the Invention
The invention relates to a rotary electric machine driving system that includes a rotary electric machine having a stator and a rotor that are arranged to face each other, a driving unit that drives the rotary electric machine and a control unit that controls the driving unit.
2. Description of Related Art
As is described in Japanese Patent Application Publication No. 2009-112091 (JP-A-2009-112091), there is known a rotary electric machine in which rotor coils are provided for a rotor and induced currents are generated in the rotor coils by a revolving magnetic field to cause the rotor to generate torque. The revolving magnetic field is generated by a stator, and includes space harmonics. In addition, with this rotary electric machine, induced currents are efficiently generated in the rotor coils to make it possible to obtain the effect of efficiently increasing torque that acts on the rotor. FIG. 23 to FIG. 25 show the schematic configuration of the rotary electric machine described in JP-A-2009-112091. FIG. 23 is a view that shows the schematic configuration of a stator and rotor when viewed in the direction parallel to the rotation axis of the rotor. FIG. 24 shows the schematic configuration of the stator. FIG. 25 shows the schematic configuration of the rotor.
However, in the case of the rotary electric machine 10 shown in FIG. 23 to FIG. 25, there is still room for improvement in terms of effectively increasing torque during low-speed rotation where the rotation speed of the rotary electric machine 10 is low. FIG. 26 is a graph that shows an example of the correlation between a rotor rotation speed and a motor torque in a range in which the rotation speed is low when the same configuration as that of the rotary electric machine shown in FIG. 23 to FIG. 25 is used as an electric motor (motor). As shown in FIG. 26, the motor torque of the rotary electric machine 10 significantly decreases in the range in which the rotation speed is low. This is because, when description will be made with reference to FIG. 23 to FIG. 25, in the rotary electric machine 10, rotor induced currents that flow through rotor coils 18n and 18s are generated by magnetic field fluctuations due to the harmonic components of the revolving magnetic field generated by a stator 12, while magnetic fluxes that link with the rotor coils 18n and 18s do not change significantly in the range in which the rotation speed is low but the fluctuation velocity of linked magnetic fluxes decreases, so induced electromotive voltages decrease to reduce the rotor induced currents. Therefore, the motor torque reduces during low-speed rotation. Note that, in the above description, the motor torque decreases when the rotary electric machine 10 is used as an electric motor in the range in which the rotation speed is low; however, when the rotary electric machine 10 is used as a generator as well, regenerative torque may significantly decrease in the low-rotation speed range because of the same reason.